1. Field of the Invention
The present invention relates to a control for a hot water heater. In particular, this invention is directed towards a method and apparatus for varying the temperature differential of a hot water heater to maintain a more consistent temperature throughout various hot water usage periods.
2. Discussion of the Related Technology
In a typical water heater design, cold water enters at the bottom of a hot water heater tank and hot water exits out of the top of the tank. When cold water rises to the level of a control sensor, a call for heat will be generated. When a water heater is subjected to repeated short draws of water, such that the water draw stops shortly after a call for heat is generated, the water temperature in the tank will stratify or "stack" with up to a 40.degree. F. water temperature differential from the control sensor to the top of the tank. Stacking produces a scald risk, because the water exiting the top of the tank is much hotter than the temperature setting at the control sensor.
Stacking can be reduced by setting a wide temperature differential at the hot water heater to allow more cold water into the tank before initiating a call for heat. Wide differential settings, however, also result in cold water complaints in the morning. Over a few hours of inactivity, water in a tank will destratify and cool off. If a demand for hot water occurs before the tank generates a call for heat, the water will be cool, and this may generate complaints. For example, a hot water heater is set to 120.degree. F. and the differential is set to 20.degree. F. Water in the tank may cool to 101.degree. F. overnight. If there is a demand for hot water in the morning, cool 101.degree. F. water will exit the top of the tank and cause complaints.
A narrower differential setting, on the other hand, decreases energy efficiency, because a water heater turns on more often when a narrow differential is set. Setting a hot water heater to a higher temperature does not decrease energy efficiency, however, it greatly increases the risk of scalding. Thus, there is a need for a water heater control that reduces stacking effects in a water heater yet maintains a comfortable hot water temperature.